1:8:25 Justifying Humiliation?

I'm a couple of days late with this, but I was struck by a piece of this section.

Part of this lengthy section elaborates on the reversal of Haman's expectation that he would receive the honors of riding on the king's horse, garbed in the king's robe, etc. The elaboration, including details not in the biblical text, accentuates the humiliation of Haman, and shows Mordechai as adding to that humiliation. For example: Mordechai insists that he cannot don the royal garments until he bathes and has his hair cut. He cannot find a bath attendant or a barber, and Haman is compelled to be bath attendant, personal butler, and (bringing his own pair of scissors) barber. We infer that all of these are low-status occupations. When Haman groans over his plight, Mordechai taunts him: "Scoundrel, didn't I know your father, who was bath attendant and barber in the village of Kiryanus for 22 years, and those are his scissors!"

So far, Mordechai's adding to Haman's grief is edgily humorous rather than morally troubling. Then, Mordechai is ready to mount the horse and says, "I'm old, and the fasting has weakened me," and Haman bends down for Mordechai to use him as a step-stool to mount the horse. Mordechai steps on him, gets up, and gets on the horse. But then the midrash adds the last detail -- in his mounting, Mordechai kicks Haman, apparently gratuitously. Haman objects, quoting Scripture: "Mordechai, 'At the downfall of your enemy, don't rejoice (Prov. 24:17).'" And Mordechai responds, "Scoundrel, '[Your enemies shall come cringing before you] And you shall tread on their backs (Deut. 33:29).'"

This last part is perturbing. Haman's plea has some appeal -- gratuitous humiliation of even our enemies seems wrong on its face, and the Scriptural citation strengthens Haman's claim on our sympathy. Yet the midrash seems to present Mordechai's action as justified, and it backs up this presentation with its own Scriptural citation (even a Torah citation, which trumps the Writings citation adduced by Haman).